The present invention relates to the production of yogurt and, in particular, to a process for the production of liquid yogurt with excellent physical stability and smooth consistency and texture.
1. Field of the Invention
Yogurt comprises a cultured milk product produced by culturing one or more dairy ingredients such as whole milk, partially skimmed milk, skim milk, nonfat dry milk and the like with a characterizing bacterial culture containing Lactobacillus bulgaricus, a lactic acid-forming rod-shaped bacterium, and Streptococcus thermophilus, a coccus-shaped bacterium able to grow and produce lactic acid at high temperatures. The dairy ingredients are first homogenized and pasteurized at high temperatures and then cooled to about 40.degree. C. to 50.degree. C. for inoculation with the culture to a desired acid content and pH at which curdling or coagulation occurs. Acid development and bacterial growth are arrested by cooling the mixture, generally to a temperature of about 0.degree. C. to about 5.degree. C. Additives such as sugar, fruit, colorants, flavorants and stabilizers may be employed to enhance consumer acceptance of the product.
Yogurts may be of the "firm" or "gel-like" variety or they may be frozen and may be eaten with a spoon. Yogurts also may be made in liquid form and drunk directly or through a straw.
Presently, nearly all yogurt in the United States is rendered "firm" or "gel-like," whereby the yogurt is processed, packaged and marketed so as to present a product having pudding-like consistency. Firm yogurts sometimes exhibit, after a few days standing, a syneresis or bleeding of whey from the body of the yogurt, typically resulting in the presence of a small amount of clear liquid on top of and around the yogurt. This bleeding can be minimized or entirely avoided by increasing the amount of milk solids in the yogurt or by adding suitable stabilizers which increase the firmness of the yogurt.
In general, liquid yogurts are prepared by using methods employed in making firm yogurts with modifications as to, inter alia, the components of the mixture, the severity of heat processing during pasteurization, increased agitation of the inoculated mixture and the like.
Liquid yogurts present particularly difficult problems with respect to stabilization. Coagulated milk proteins, suspended fruit particles and the like will, only a short time after preparation, separate completely from the product and form either a sediment at the bottom of the container, a layer floating at the top of the product or a combination of these undesirable effects. This is attributed to their thin, pourable consistency.
Attempts at overcoming the difficulties described above generally involve resort to techniques utilized in the manufacture of firm yogurts, such as the addition of stabilizers. Many liquid yogurt products have a shelf life expectation of two to three weeks. However, a shelf life of five or six weeks is commercially desirable. It is therefore necessary to stabilize the liquid yogurt using commercial stabilizers. These include plant exudates (e.g., gum arabic), seaweed extracts (e.g., alginates), plant and seed gums (e.g., guar gum), animal derivatives (e.g., gelatin), plant extracts (e.g., pectin). These commercial stabilizers are completely soluble in milk or already prepared yogurt, and may, therefore, be incorporated at any stage of manufacture. However, such large quantities of stabilizers are required to prevent the separation of milk proteins and suspended particles in liquid yogurt that the resultant product is undesirably viscous and/or exhibits off flavors, objectionable color, or an objectionable grainy or chalky texture.
Although the prior art suggests high pressure homogenization of yogurt as a means for obtaining a product of acceptable texture and consistency, such homogenization is difficult and expensive to operate in a sanitary manner and requires that the product be sterilized thereafter, adding yet additional expense. Moreover, pasteurized yogurt products, in which the bacterial culture is completely destroyed, generally are not considered true yogurts.
We have observed that excellent mouthfeel and stability in liquid yogurt as well as special compatability with fruit preparations may be obtained when high methoxyl pectin is incorporated in liquid yogurt. However, these are several factors that make high methoxyl pectin difficult to use in commercial practice.
For example, if high methoxyl pectin is added to milk before pasteurization or at any stage before the yogurt is at pH higher than about 4.6, the pectin will precipitate the milk protein; a sediment will form on the bottom and whey on the top. After mixing the milk with culture, no further agitation can take place if a typical yogurt is to be produced. The culturing of a mixture of precipitate and whey leads to a type of yogurt which is gritty in mouthfeel and completely unsatisfactory. If the high methoxyl pectin is added when pH is lower than 4.6, no precipitation occurs. A product very satisfactory in stability and mouthfeel is obtained, as long as the pectin is incorporated in the yogurt at the appropriate time.
2. Discussion of the Prior Art
High methoxyl pectin has been used to stabilize milk beverages acidified by the addition of edible acids or fruit preparations. In these cases, the pectin and milk mixture is agitated continuously so that a pectin-protein precipitate or sediment will not form. When the mixture is then acidified by edible acids or acid fruit preparations, the pectin and protein no longer act to precipitate each other and the pectin stabilizes the milk protein. Such acidified milk beverages, it should be noted, are quite distinct from yogurt in rheological properties.
Stabilization of said acidified milk beverages is described in Dutch Pat. No. 117,194, July 1952 and in an article by G. V. Gudnason, et al., published in American Milk Review, April 1960. The Dutch patent describes a process which comprises pasteurizing or sterilizing the beverages. Such heat treatment solubilizes the pectin so that it will completely stabilize the proteins. In the experiments described by Gudnason, et al., no heat treatment was employed. The results of these experiments showed that the pectin-milk mixture formed a sediment after 48 hours. In the above references there was no homogenization step after the pectin addition. It is believed that heat treatment alters the form of calcium in the milk which enables the pectin to dissolve.
Information booklet (Genu Service 15-01) issued by the Copenhagen Pectin Factory Ltd. in April 1980 states that high methoxyl pectin is not soluble in yogurt due to the form in which calcium is present. High shear forces must be applied to the yogurt-pectin dispersion so that the pectin will properly stabilize the yogurt. It is suggested that this is best achieved by passing the mixture of yogurt and pectin through a homogenizer at a pressure of 1500-2500 psi.
A quality control method for viscosity of high methoxyl pectin issued by the Pectin Factory Ltd. on June 12, 1979, discloses that after thoroughly mixing the pectin and yogurt, the mixture is homogenized at 2250 psi, and that this homogenization will bring the pectin into solution.
Further examples of the use of pectin to stabilize sour milk drinks are described in U.S. Pat. No. 3,625,702 to Exler and U.S. Pat. No. 3,978,243 to Pedersen. Exler discloses the stabilization of sour milk drinks by adding high methoxyl pectin to acid milk and homogenizing the mixture at about 5000 psi prior to pasteurization at an elevated temperature. Pederson describes the manufacture of a gelled sour milk product, wherein high methoxyl pectin, carboxymethylcellulose, or propylene glycol alginate are used to stabilize the sour milk prior to gelling it by electronegatively charged gelling agents such as low methoxyl pectin, carrageenan, or furcellaran. The preferred procedure employs homogenization at 2250 to about 5000 psi when high methoxyl pectin is employed.
Unpasteurized liquid yogurts are expected to have a relatively short shelf life of about two to three weeks. Said products must be sold or utilized before stability problems or microbiological spoilage occurs. Microbiological spoilage generally is caused by yeast and mold contamination during production. The homogenizer is a major source of such contamination because it is so difficult to sanitize. This problem may be prevented by pasteurizing or sterilizing the product as a last step in the processing. However, since this step not only destroys yeasts and molds but also the Lactobacilli and Streptococci, the resultant products may not be considered true yogurts by most persons skilled in the art. A homogenizer can be completely sterilized before use under laboratory conditions, but in commercial practice this is rarely possible.
The present invention satisfies a long felt need to develop a process in which high methoxyl pectin may be incorporated into already prepared yogurt, rendering it physically stable without the need to use high pressure homogenization and subsequent pasteurization or sterilization.